In a hydraulic system including a hydraulic gear motor, it is desirable to provide for fluid pressure relief in the event that the load on the motor becomes excessive. Without pressure relief, an excessive load or a stalled motor may cause damage to system components. In one arrangement, the fluid inlet and outlet ports to the motor are connected by a cross-over relief valve. This is a normally closed valve connected between the ports. The valve also has a pilot port which is connected to the inlet port to the motor. When a predetermined pressure is exceeded at the pilot port, the valve is opened to allow hydraulic fluid to bypass the motor.
For a bi-directional motor, either port may be the inlet port, depending upon the direction in which the motor is to be driven. This type of motor requires two cross-over relief valves mounted in parallel between the two motor ports. Each valve is responsive to the pressure on a different port. In another arrangement, a single cross-over valve is connected through four check valves to the two motor ports. The check valves allow the highest pressure fluid at the two motor ports to flow to the inlet of the cross-over valve and allow the output from the cross-over valve to flow to the motor port having the lowest pressure. Both of these arrangements require more valves than are necessary to protect the motor.
A hydraulic gear pump may have substantially the same design as a hydraulic gear motor. The only difference is that rotary motion is converted to fluid pressure in the pump, while fluid pressure is converted to rotary motion in the motor. Cross-over relief valves have been used to limit the fluid pressure of a pump in the same manner in which such valves limit fluid pressure in a motor. For a bi-directional hydraulic pump, two cross-over relief valves are required, as with the bi-directional motor.
Many hydraulic motor driven systems have a normally set hydraulic brake. The brake is of the pressure release type and is released only after sufficient hydraulic pressure is applied to the motor. Such a system is used, for example, in hydraulically operated elevators and for hydraulically operated excavation equipment. Since the motors are bi-directional, the brake must be released in response to the highest pressure at either of two fluid ports on the motor. Typically, the two ports are connected through individual check valves to the brake release valve. Or, the two ports may be connected to two inputs to a single shuttle valve which in turn operates the brake release valve. In either arrangement, the valves are more expensive than is necessary.